In installations for the distillation of air, atmospheric air is compressed to 6 bars absolute by a several-stage compressor. Each intermediate stage comprises an intermediate heat exchanger, a so-called "interstage cooler", and the last stage comprises a heat exchanger called a "final cooler". These exchangers are generally supplied by water which comes from the air-cooled apparatus, which treats the water returned from the exchangers.
Because of the evaporation of a portion of the water in the refrigeration apparatus and the need to effect purges of the circuit, this apparatus is supplied with a makeup water flow, which usually comes from an underground water supply.
The water treated by the cooling apparatus will be at a temperature that varies seasonally, as a function of the air temperature. At least in the warm season, it will generally not permit lowering the air temperature from the last compressor stage below 25.degree. to +30.degree. C. To optimize the apparatus for purification by adsorption by reducing the quantity of adsorbent needed, there is positioned between the final cooler and the adsorption apparatus a refrigeration group, or another auxiliary cooling apparatus, so as to lower the temperature of the compressed air, typically to below +15.degree. C.
The air distillation systems generally comprise other compression apparatus, also cooled by water from the mentioned circuit: an air supercharger mounted downstream of the principal compressor, generally coupled to an air expansion turbine, and/or a nitrogen cycle compressor. These compression apparatus generally feed cryogenic cooling exchangers, and it would be interesting to precool in a more rapid way the gas which they receive, for example to increase the production of liquid.
However, in these compression apparatus as in the principal air compressor, the reduction of the temperature of the compressed gas, at least in the warm season, below about 25.degree. C., requires the use of a refrigeration group or of another auxiliary apparatus, whose cost and maintenance expense is not negligible.